The Collaborative International Dictionary of English v.0.48:

Decorum \De*cor"um\, n. [L. dec[=o]rum, fr. dec[=o]rus. See
   Decorous.]
   Propriety of manner or conduct; grace arising from
   suitableness of speech and behavior to one's own character,
   or to the place and occasion; decency of conduct; seemliness;
   that which is seemly or suitable.
   [1913 Webster]

         Negligent of the duties and decorums of his station.
                                                  --Hallam.
   [1913 Webster]

         If your master
         Would have a queen his beggar, you must tell him,
         That majesty, to keep decorum, must
         No less beg than a kingdom.              --Shak.

   Syn: Decorum, Dignity.

   Usage: Decorum, in accordance with its etymology, is that
          which is becoming in outward act or appearance; as,
          the decorum of a public assembly. Dignity springs from
          an inward elevation of soul producing a corresponding
          effect on the manners; as, dignity of personal
          appearance.
          [1913 Webster]

WordNet (r) 3.0 (2006):

decorum
    n 1: propriety in manners and conduct [syn: decorum,
         decorousness] [ant: indecorousness, indecorum]